1. Field of the Invention
The present invention relates to an amplifier. In particular, the invention relates to the technology for improving the frequency characteristic of a differential amplifier.
2. Description of the Related Art
Conventionally, differential amplifiers for amplifying input signals that are input in a differential manner have been known (for example, see David Johns and Ken Martin “ANALOG INTEGRATED CIRCUIT DESIGN,” John Wiley & Sons, Inc., 1997, p. 274). FIG. 4 shows the configuration of a conventional differential amplifier. A first transistor Tr100 and a second transistor Tr102 are connected at their gates to a first input terminal IN100, or noninverting input terminal, and a second input terminal IN200, or inverting input terminal, respectively. The common source of the transistors Tr100 and Tr102 is grounded via a constant current source Iamp100.
The drains of the first transistor Tr100 and the second transistor Tr102 are connected to the drains and gates of a third transistor Tr104 and a fourth transistor Tr106, respectively. The common source of the third transistor Tr104 and the fourth transistor Tr106 is connected to a voltage source V100. The gates of the transistors Tr104 and Tr106 are connected to the gates of a sixth transistor Tr110 and a fifth transistor Tr108, respectively, to constitute current mirrors. The common source of the fifth transistor Tr108 and the sixth transistor Tr110 is connected to the voltage source V100. The drains of the sixth transistor Tr110 and an eighth transistor Tr114 are connected to an output terminal Iout100. The drain of the fifth transistor Tr108 is connected to the drain and gate of a seventh transistor Tr112. The common source of the seventh transistor Tr112 and the eighth transistor Tr114 is grounded. The gates of the transistors Tr112 and Tr114 are connected to each other to constitute a current mirror.
When a positive voltage is applied to the gate of the first transistor Tr100 through the first input terminal IN100, a current flows through the first transistor Tr100 due to the bias current of the constant current source Iamp100. This lowers the voltage of the drain and gate of the third transistor Tr104. The third transistor Tr104 passes the same current as that flowing through the first transistor Tr100. The gate voltage at this point is given to the gate of the sixth transistor Tr110, and the sixth transistor Tr110 also passes a current. The gate width of the sixth transistor Tr110 is designed to pass a current n times as high as that flowing through the third transistor Tr104. Consequently, the source current obtained by amplifying the current flowing through the first transistor Tr100 n times appears on the output terminal Iout100.
When a positive voltage is applied to the gate of the second transistor Tr102 through the second input terminal IN200, a current flows through the second transistor Tr102 due to the bias current of the constant current source Iamp100. This lowers the voltage of the drain and gate of the fourth transistor Tr106. The fourth transistor Tr106 passes the same current as that flowing through the second transistor Tr102. The gate voltage at this point is given to the gate of the fifth transistor Tr108, and the fifth transistor Tr108 also passes the same current as that flowing through the fourth transistor Tr106. When the drain and gate of the seventh transistor Tr112 drop in voltage, the same current as that flowing through the fifth transistor Tr108 flows through the seventh transistor Tr112. The gate voltage at this point is given to the gate of the eighth transistor Tr114, and the eighth transistor Tr114 also passes a current. The gate width of the eighth transistor Tr114 is designed to pass a current n times as high as that of the seventh transistor Tr112. Consequently, a sink current n times as high as the current flowing through the second transistor Tr102 appears on the output terminal Iout100.
Conventional differential amplifiers have the characteristic that their gains decrease with the increasing frequencies of signals to be amplified. FIG. 5 shows the relationship between the operating frequency and the gain of a conventional differential amplifier. For example, in a differential amplifier having a gain of g [dB], signals having frequencies no higher than f can be amplified at the gain g. Signals exceeding the frequency f can only be amplified at amplifications lower than the gain g. That is, if the input signal contains high frequency components beyond the operating frequency range, the ratio between the amount of the input current flowing in response to the input voltage and the amount of the output current (hereinafter, referred to as amplification efficiency) drops as a whole. It has thus been the case that the amplification efficiency drops when differential amplifiers are designed for higher gains. On the other hand, higher gains have had to be sacrificed in order to design for higher amplification efficiencies.